Method and device for separating a substance out of a solution

A method for separating off a substance by crystallization from a solution of the substance, in which

• a suspension of seed crystals is produced and, when a desired amount of seed crystals is present, a crystallization method is started, in which crystals of the substance are obtained which are then separated off, where, for producing the desired amount of seed crystals:
• electromagnetic radiation is radiated into the solution, where the electromagnetic radiation radiated into the solution has the form of a beam, an aperture angle of which is greater than 5 degrees,
• for establishing a desired amount of seed crystals an intensity of the electromagnetic radiation which has been scattered by crystals located in the solution is detected and then the crystallization method is started,
• the detected intensity is compared with a desired intensity (IS),
• the temperature of the solution is regulated depending on any difference between the detected intensity and the desired intensity (IS) in such a way that an amount of this difference is reduced,
• if the amount of the difference between the detected intensity and the desired intensity (IS) is less than a limiting value, the desired amount of seed crystals for the crystallization method is present, wherein the desired intensity (IS) is determined by reference measurements by which, for the solution, the relationship between the crystal size and/or the crystal morphology at an end of the crystallization method and the detected intensity at the start of the crystallization method is determined and from this the desired intensity (IS) is selected as the intensity for the desired crystal size and/or crystal morphology.